1. Field of the Invention
The present invention relates to a method for manufacturing an SOI substrate which has a semiconductor layer formed over an insulating film and a semiconductor device using the SOI substrate.
2. Description of the Related Art
In recent years, instead of a bulk silicon wafer, integrated circuits using an SOI (silicon on insulator) substrate have been developed. By utilizing characteristics of a single crystal silicon thin film formed over an insulating film, semiconductor layers of transistors formed in the integrated circuit can be separated from each other completely. Further, since the fully-depleted transistors can be formed, a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low power consumption can be realized.
Known examples of SOI substrates are SIMOX substrates and bonded substrates. For example, an SOI structure of a SIMOX substrate is obtained in such a manner that oxygen ions are implanted into a silicon wafer and thermal treatment is performed at 1300° C. or higher to form a buried oxide (BOX) layer, so that a single crystal silicon thin film is formed on the surface of the substrate.
In the case of a bonded substrate, an SOI structure is obtained in such a manner that two silicon wafers (a base substrate and a bond substrate) are bonded to each other with an oxide film interposed therebetween, and one of the silicon wafers (the bond substrate) is thinned by grinding and/or polishing on its rear side (the side which is not a surface bonded), so that a silicon thin film is formed. Meanwhile, there is proposed a technique called an ion implantation separation method which employs hydrogen ion implantation (e.g., Reference 1: Japanese Published Patent Application No. H5-211128), because it is difficult to form a uniform silicon thin film by grinding or polishing. The ion implantation separation method is also called a Smart Cut (registered trademark).
Hereinafter, an outline of the ion implantation separation method will be described. First, hydrogen ions are implanted into a silicon wafer by an ion implantation method, so that an embrittlement region is formed at a predetermined depth from a surface of the silicon wafer. Next, another silicon wafer which serves as a base substrate is oxidized to form a silicon oxide film. After that, the silicon wafer into which hydrogen ions are implanted is made to have contact with the silicon oxide film of the silicon wafer which serves the base substrate, so that the two silicon wafers are bonded to each other. Then, through thermal treatment, the silicon wafer is separated using the embrittlement region as a cleavage plane, thereby forming a substrate in which a thin film of single crystal silicon is bonded to the silicon wafer which serves as the base substrate.
As for an ion implantation method, particles to be implanted into a sample are ionized in vacuum and accelerated by the electric field. An ion implanter used for an ion implantation method includes an ion source, a mass separation unit, an acceleration unit, a beam scanner (electrostatic scan) unit, an implantation chamber (end-station), and an evacuation unit. Further, since a cross section of an ion beam is not uniform, scanning with an ion beam is performed on a surface of the sample in order to obtain uniformity.
One of the known examples of semiconductor devices using SOI substrates is disclosed (see Reference 2: Japanese Published Patent Application No. 2000-012864). In Reference 2, a method for manufacturing an SOI substrate, in which hydrogen is implanted by an ion implantation method and a high heat-resistance substrate is used, is disclosed.